SE-B-514 774 discloses a centrifugal separator of the kind initially defined. As appears from this document, it could be difficult to maintain the interface layer level at the desired radial position during operation of the centrifugal separator. This can be due to the fact that a non-controllable quantity of separated heavy phase, including separated solid particles, are discharged per time unit. If the discharged quantity of heavy phase, for instance would exceed a quantity of fed heavy phase, the interface layer level will be radially displaced outwardly. This problem is solved in SE-B-514 774 by means of a control equipment comprising separate members for supply and discharge of a control fluid which has a higher density than the light phase.
A common separation case is that the heavy phase is controlled in the manner mentioned above in such a way that the counter pressure in the outlet of the heavy phase is maintained at a determined level and that the light phase flows over an overflow outlet. In such a separation case, it may occur that the interface layer level is displaced to an undesired radial position due to the gas pressure prevailing at the free liquid surface adjacent to the overflow outlet. Such a displacement of the interface layer level may lead to a poor separation and/or breaking of the water seal.
In a centrifugal separator, including a paring disc with venting holes and atmospheric pressure outside the bowl, this problem will not arise. The actual gas pressure is then the atmospheric pressure, which can be regarded as constant. This problem does not occur also when there is the conventional configuration with a flow over an overflow outlet for the heavy phase and over an overflow outlet for the light phase, wherein the radial levels of both the overflow outlets control the radial position of the interface layer level. If this configuration comprises a paring disc with venting holes for the light phase, the same gas pressure prevails at the free liquid surface adjacent to the overflow outlets both for the heavy phase and the light phase, which means that the interface layer level will not be influenced by variations in the gas pressure.
However, if one of the phases is controlled with respect to the counter pressure, a variation in the gas pressure will influence directly the radial position of the interface layer level if corresponding compensation of the counter pressure is not made on the phase controlled with respect to the counter pressure. Variations in the gas pressure adjacent to the overflow outlet arise when the gas adjacent to the overflow outlet lacks a free flow path for pressure equalization. The variations of the gas pressure become large especially when the product to be separated and to be supplied to the centrifugal separator has a high steam pressure, i.e. an oil-water mixture, which is saturated with natural gas and which has a temperature close to the boiling point of the water phase.